DEAR DR. ROACH: My stepfather had a series of screening tests done by a commercial company, including tests on his carotid arteries, abdominal aorta, peripheral arteries and heart rhythm. He received a good report. However, two or three weeks after the screening, he suffered a stroke. My confidence in the screening program was shaken! How accurate are these tests? — R.M.

ANSWER: I am sorry about your stepfather, and hope he recovers fully.

Even if the tests were performed with the best techniques available, screening tests are unable to identify all risks, especially for stroke. Many strokes come from plaque in the major arteries, especially the carotid arteries, which provide the blood supply to the brain, and are part of the screening. The plaque can also be in the arch of the aorta, which is the blood vessel that supplies all the blood to the body. The arch is the first part of this major blood vessel, and the carotid arteries come off the aorta in the arch.

However, strokes can come from diseased blood vessels inside the brain, which are also difficult to see. Heart rhythm problems such as atrial fibrillation, which can come and go and might not be picked up on during the screening, can also lead to stroke, as can blood clots elsewhere in the body that go through a septal defect (“hole”) in the heart and from there to the brain.

Screening tests can miss several potential sources of stroke. This can lead to an inappropriate sense of security, which could keep a person from seeking medical attention quickly in the event of symptoms. Even for areas that the screening test can see, such as the carotid arteries, it is possible to have a false negative test — disease is present but isn’t registered by the test.

The other problem with screening tests, especially in those done in people with low risks, is that the test can say there is a problem when there isn’t. This is called a false positive test, and can lead to unnecessary follow-up testing and a great deal of worry.

After a stroke, doctors try hard to identify the underlying cause, so it can be treated if possible. Your stepfather should have undergone a thorough evaluation. If a cause is identified, it would be appropriate to see whether the screening test was wrong or if the stroke was from a cause that was not looked for in the battery of screening tests he got.

There are some people who have strokes for which no cause can be identified. People in this category should still pay careful attention to stroke risk factors, including blood pressure, cholesterol and smoking cessation, if appropriate. Many people will be put on medication, such as aspirin or a statin. Heavy drinkers should cut down their alcohol use. Although often overlooked, regular moderate exercise and a healthy diet, such as a Mediterranean diet, have been shown to reduce risk of stroke and heart attack, which is particularly important in a person who has already had one.

DEAR DR. ROACH: Are there any trace amounts of mercury in flu shots? — D.K.

ANSWER: There is no metallic mercury in flu shots. A preservative called thiomersal is used in some flu shots (those in multidose vials), and this contains a small amount of organic mercury called ethyl mercury. The body is able to excrete ethyl mercury within weeks. Expert groups, such as the World Health Organization, have concluded that the tiny amount of ethyl mercury in flu vaccines is safe.

However, some people prefer not to get any preservative, so single dose vaccines are available. Those are preservative-free.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

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